U.S. patent number 6,181,450 [Application Number 09/076,494] was granted by the patent office on 2001-01-30 for system and method for free space optical communications.
This patent grant is currently assigned to Harris Corporation. Invention is credited to Edward R. Beadle, John F. Dishman.
United States Patent |
6,181,450 |
Dishman , et al. |
January 30, 2001 |
System and method for free space optical communications
Abstract
A method and system of communicating in free space using an
optical communication system, such as for intersatellite and
satellite-to-ground communications, is disclosed. Digital
communication signals are multiplexed with a plurality of other
analog communication signals into a single broad band frequency
division multiplexed signal. A laser generates an optical carrier
and an electro-optic modulator modulates the optical carrier signal
with the frequency division multiplexed signal to produce a phase
modulated optical communications signal. A receiver is positioned,
such as in a satellite, to receive the phase modulated optical
communications signal. The receiver includes a demodulator for
demodulating the phase modulated optical communication system back
into the broad band frequency division multiplexed signal and a
demultiplexer for demultiplexing the broad band frequency division
multiplexed signal into the plurality of communication signals. The
portion of those signals that were previously digital data can be
demodulated back into the digital communication signals.
Inventors: |
Dishman; John F. (Palm Bay,
FL), Beadle; Edward R. (Melbourne, FL) |
Assignee: |
Harris Corporation (Palm Bay,
FL)
|
Family
ID: |
22132395 |
Appl.
No.: |
09/076,494 |
Filed: |
May 12, 1998 |
Current U.S.
Class: |
398/122; 370/281;
370/319; 398/79; 398/9 |
Current CPC
Class: |
H04B
10/118 (20130101); H04B 7/18521 (20130101); H04B
7/18513 (20130101); G02F 1/291 (20210101); G02F
1/292 (20130101) |
Current International
Class: |
H04B
7/185 (20060101); H04B 10/105 (20060101); G02F
1/29 (20060101); H04J 014/02 (); H04B 010/00 () |
Field of
Search: |
;359/172,124,130,159,183
;370/281,319 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Allen, Dyer, Doppelt, Milbrath
& Gilchrist, P.A.
Claims
That which is claimed is:
1. A method of communicating in free space comprising the steps
of:
frequency division multiplexing a plurality of communication
signals into a single broad band frequency division multiplexed
signal;
phase modulating an optical carrier signal with the broad band
frequency division multiplexed signal by mixing the multiplexed
signal with the optical carrier signal in an electro-optic
modulator to produce a phase modulated optical communications
signal;
transmitting the phase modulated optical communications signal to a
receiver;
demodulating the phase modulated optical communications signal back
into the broad band frequency division multiplexed signal; and
demultiplexing the broad band frequency division multiplexed signal
into the plurality of analog communication signals.
2. A method according to claim 1, and further comprising the steps
of generating a plurality of digital communication signals and
modulating the digital communication signals into digital waveform
communication signals before frequency division multiplexing.
3. A method according to claim 1, and further comprising the step
of phase modulating the optical carrier signal by mixing the
optical carrier signal and the broad band frequency division
multiplexed signal within a Mach-Zender electro-optic
modulator.
4. A method according to claim 1, and further comprising the steps
of up converting each of the plurality of analog communication
signals into a defined frequency slot, and combining the
frequencies to create the frequency division multiplexed
signal.
5. A method according to claim 1, and further comprising the step
of non-mechanically steering the phase modulated optical
communications signal by passing the communications signal through
a Bragg cell and liquid crystal display.
6. A method according to claim 1, and further comprising the step
of beam spoiling the phase modulated optical communications signal
by increasing the beam convergence of the communications signal
during transmission.
7. A method according to claim 1, and further comprising the step
of demodulating the phase modulated optical communications signal
by phase locked loop detection.
8. A method of communicating in free space comprising the steps
of:
frequency division multiplexing a plurality of communication
signals into a single broad band frequency division multiplexed
signal;
generating with a laser an optical carrier signal;
phase modulating the optical carrier signal with the broad band
frequency division multiplexed signal by mixing the multiplexed
signal with the optical carrier signal in an electro-optic
modulator;
transmitting the phase modulated optical communications signal to a
receiver while non-mechanically steering the optical communications
signal by passing the optical communications signal through a Bragg
cell and liquid crystal display to provide for fine indexing of the
signal in the liquid crystal display and coarse two-dimensional
indexing in the Bragg cell;
demodulating the phase modulated optical communications signal back
into the broad band frequency division multiplexed signal; and
demultiplexing the frequency division multiplexed signal into the
plurality of analog communication signals.
9. A method according to claim 8, and further comprising the steps
of applying a radio frequency signal to the Bragg cell to change
the index of refraction for steering.
10. A method according to claim 8, and further comprising the steps
of passing the generated optical communications signal through
first and second Bragg cell members for providing two dimensional,
coarse steering.
11. A method according to claim 8, and further comprising the steps
of generating a plurality of communication signals and modulating
the digital communication signals into digital waveform
communication signals.
12. A method according to claim 8, and further comprising the steps
of phase modulating the optical carrier signal by mixing the
optical carrier signal and the broad band frequency division
multiplexed signal within a Mach-Zender electro-optic
modulator.
13. A method according to claim 8, and further comprising the steps
of up converting each of the plurality of communication signals
into a defined frequency slot, and combining the frequencies to
create the broad band frequency division multiplexed signal.
14. A method according to claim 8, and further comprising the step
of non-mechanically steering the phase modulated optical
communications signal by passing the optical carrier signal through
a Bragg cell and liquid crystal display.
15. A method according to claim 8, and further comprising the step
of beam spoiling the phase modulated optical communications signal
by increasing the beam convergence of the communications signal
during transmission.
16. A method according to claim 8, and further comprising the step
of demodulating the phase modulated optical communications signal
by heterodyne detection.
17. A system for communicating in free space comprising:
means for multiplexing a plurality of analog communication signals
into a single frequency division multiplexed signal;
a laser for generating an optical carrier signal;
an electro-optic modulator that receives said frequency division
multiplexed signal and said laser generated optical carrier signal
and phase modulates the optical carrier signal with the multiplexed
signal to produce a phase modulated optical communications
signal;
a receiver that receives the phase modulated optical communications
signal, said receiver further comprising:
a demodulator for demodulating the phase modulated optical
communications signal back into the frequency division multiplexed
signal, and
a demultiplexer for demultiplexing the broad band frequency
division multiplexed signal into the plurality of analog
communication signals.
18. A system according to claim 17, and further comprising means
for generating a plurality of communication signals and means for
modulating the digital communication signals into digital waveform
communication signals.
19. A system according to claim 17, and wherein said electro-optic
modulator further comprises a Mach-Zender electro-optic
modulator.
20. A system according to claim 17, wherein said means for
multiplexing further comprises a respective mixer for up converting
respective analog signals into a defined frequency channel, and a
combiner for combining the up converted frequencies into the
frequency division multiplexed signal.
21. A system according to claim 17, and further comprising a Bragg
cell and liquid crystal display that receives the phase modulated
optical communications signal for non-mechanically steering the
phase modulated optical communications signal.
22. A system according to claim 17, and further comprising a beam
spoiler that receives the phase modulated optical communications
signal for increasing the beam convergence of the phase modulated
optical communications signal during transmission.
23. A system according to claim 17, and further comprising a phase
lock loop detector for demodulating the phase modulated optical
communications signal.
24. A system for communicating in free space comprising:
means for multiplexing a plurality of analog communication signals
into a single broad band frequency division multiplexed signal;
a laser for generating an optical carrier signal;
an electro-optic modulator that receives said broad band frequency
division multiplexed signal and said laser generated optical
carrier signal and phase modulates the optical carrier signal with
the multiplexed signal to produce a phase modulated optical
communications signal;
steering means for non-mechanically steering the optical
communications signal, said steering means further comprising:
a Bragg cell to provide coarse two-dimensional indexing within the
Bragg cell and a liquid crystal display to provide fine indexing of
the phase modulated optical communications signal;
a receiver that receives the phase modulated optical communications
signal, said receiver further comprising:
a demodulator for demodulating the phase modulated optical
communications signal back into the broad band frequency division
multiplexed signal, and
a demultiplexer for demultiplexing the broad band frequency
division multiplexed signal into the plurality of analog
communication signals.
25. A system according to claim 24, and further comprising means
for applying a radio frequency signal to the Bragg cell to change
the index of refraction and provide steering.
26. A system according to claim 24, wherein said Bragg cell further
comprises first and second Bragg cell members to provide two
dimensional, coarse steering.
27. A system according to claim 24, and further comprising means
for generating a plurality of communication signals and means for
modulating the digital communication signals into analog
communication signals.
28. A system according to claim 24, and wherein said electro-optic
modulator further comprises a Mach-Zender electro-optic
modulator.
29. A system according to claim 24, wherein said means for
multiplexing further comprises a respective mixer for up converting
respective analog signals into a defined frequency channel, and a
combiner for combining the up converted frequencies into the
frequency division multiplexed signal.
30. A system according to claim 24, and further comprising a Bragg
cell and liquid crystal display that receives the phase modulated
optical communications signal for non-mechanically steering the
phase modulated optical communications signal.
31. A system according to claim 24, and further comprising a beam
spoiler that receives the phase modulated optical communications
signal for increasing the beam divergence of the phase modulated
optical communications signal during transmission.
32. A system according to claim 24, and further comprising a phase
locked loop detector for demodulating the phase modulated optical
communications signal.
Description
FIELD OF THE INVENTION
This patent application is related to the field of optical
communications, and more particularly, this patent application is
related to optical intersatellite and satellite-to-ground
communication systems.
BACKGROUND OF THE INVENTION
This invention describes a method by which next generation
satellite communication systems can achieve extremely high data
rates for direct intersatellite, satellite-to-ground, and
ground-to-satellite communication over extremely large
line-of-sight distances using optical technology. Traditionally,
intersatellite links have been implemented in the microwave and
millimeter wave regions. However, these options have limitations
imposed by wavelength, transmit power, and modulation bandwidth.
Optical data transmission overcomes these limitations. The small
wavelength provides extremely high gains for the required transmit
power for reliable communication at very large distances. In
addition, the modulation bandwidths achievable for optical based
channel are on the order of 20 Ghz and still rapidly increasing,
where radio-frequency technologies are experiencing only
incremental improvements. Such wide bandwidth provides a suitable
channel for data communications exceeding 20 Gbps. However, current
optical data communication technology is following the development
of terrestrial fiber optic networks and concentrating solely on the
transmission of digital information, typically using a pulse-pulse
modulation (PPM) or on-off keying (OOK) format. Also common is the
use of multiple optical wavelengths, called wavelength-division
multiplexing (WDM), to increase the information rate in an optical
channel. These techniques do not fully exploit the advantages of
optical communication technology for high data rate space-based
applications. There are several weaknesses to those approaches. For
example, the PPM technique is not suitable for high data
communications due to difficulties in detection and low bandwidth
efficiency, and OOK is typically limited to applications that can
use direct modulation of the laser. Also, WDM is not desirable,
especially for space as multiple lasers are required as size,
weight, power, cost and reliability are all degraded when adding
multiple active components such as lasers.
The present invention circumvents all of the above shortcomings by
electrically combining a number of data sources, digital or analog,
using a frequency-division multiple access scheme, and using this
signal as a wideband modulating signal to alter the phase of a
single optical carrier. The constant envelope of phase modulation
is advantageous as compared to amplitude modulations (OOK, PPM) for
simplifying detection schemes as is well known in communications.
At the receiving terminal, the carrier is coherently demodulated
and the individual electrical signals recover using filtering and
amplification. This invention, unlike known prior art, allows
digital and analog signals to simultaneously share a single optical
carrier.
Some prior art systems have used optical communication systems to
an advantage. For example, U.S. Pat. No. 5,610,748 to Sakanaka et
al. discloses a communications link having intensity modulation
with a necessary pilot (e.g., auxiliary) signal. Intensity
modulation is also more difficult in a system where the transmitter
and receiver are moving relative to each other, such as with moving
satellites. Also, the laser beam intensity changes with the
distance between the transmitter and receiver, such as when
satellites orbit, causing some data inconsistencies because the
laser attenuation appears as a change in a data bit. Thus,
intensity modulation is not as desirable as constant envelope
modulation (i.e., phase modulation), for most free space
communications. Although intensity modulation has been successfully
used in some optical transmission systems, using a fiber system as
disclosed in U.S. Pat. No. 5,351,148 to Maeda et al. However, it is
desirable if another modulation besides intensity modulation were
used for optical communications in free space. Additionally,
because of the moving transmitter and receiver in intersatellite
communication systems, more conventional mechanical steering
elements are difficult to operate and it would be advantageous if a
non-mechanical steering system could be used with such systems.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
method and system of communicating in free space with
intersatellite and satellite-to-ground communications that do not
use intensity modulation or WDM of an optical carrier.
It is still another object of the present invention to provide a
method and system of communicating in free space with an optical
carrier signal produced by a laser that allows both analog and
digital data to be sent simultaneously on the same optical carrier
signal.
The present invention is advantageous because it now allows both
analog and digital data to be transmitted simultaneously on a phase
modulated optical communication signal to a receiver, such as for
intersatellite and satellite-to-ground communications. Because the
optical carrier signal is phase modulated, the problems associated
with moving targets and changes in distances between the targets,
e.g., satellites and/or ground stations and satellites, are
reduced. The system and method of the present invention uses a
constant envelope type of modulation, i.e., phase modulation,
instead of the more conventional intensity modulation, which
changes the amplitude of the signal. As a result, no auxiliary or
pilot signal is necessary. Additionally, the phase modulated signal
is readily adapted for non-mechanical steering which decreases any
payload weight for communications equipment, requires less fuel and
decreases acquisition times.
In accordance with the present invention, the system and method of
the present invention allows communication in free space, such as
in intersatellite communications, and includes a frequency division
multiplexer for multiplexing a plurality of analog communication
signals into a single broad band frequency division multiplexed
signal.
Typically, a mixture of analog, digital or RF are each passed
through a mixer where respective signals are up converted into a
unique signal slot or channel. The frequencies then are combined to
form the broad band frequency division multiplexed signal. A laser
generates an optical carrier signal. An electro-optic modulator
phase modulates the optical carrier signal with the multiplexed
signal to produce a phase modulated optical communication
signal.
A receiver is positioned, such as in a satellite, to receive the
phase modulated optical communications signal. The receiver
comprises a demodulator for demodulating the phase modulated
optical communications signal back into the original broad band
frequency division multiplexed signal. A demultiplexer (e.g.,
filter) allows demultiplexing of the broad band frequency division
multiplexed signal into the plurality of communication signals
comprising the frequency division multiplexed signal.
In still another aspect of the present invention, a plurality of
digital communication signals are generated and analog modulated
onto an optical carrier using electro-optic technique. The
electro-optic modulator can preferably comprise a Mach-Zender
electro-optic modulator. An antenna can receive communication
signals to be multiplexed with a receiver, such as in a satellite,
and can be connected through the frequency division multiplexer for
receiving analog communication signals generated by a remote
source. The electro-optic modulator preferably generates an optical
carrier signal wavelength of about 1,550 nm. This wavelength is
preferable because erbium-doped fiber amplifiers can be used at
this wavelength for amplifying the phase modulated optical
communication signals.
The system further comprises a Bragg cell and a liquid crystal
display that receives the phase modulated optical communication
signal for non-mechanically steering the phase modulated optical
communication signal. The system also comprises a beam spoiler that
receives the phase modulated optical communication signal for
increasing the beam divergence of the phase modulated optical
communication signal during transmission.
In still another aspect of the present invention, a non-mechanical
steering device includes a Bragg cell to provide coarse
two-dimensional indexing and a liquid crystal display to provide
fine indexing of the phase modulated optical communications signal.
The Bragg cell can further comprise first and second Bragg cell
members to provide the two-dimensional coarse steering. The first
and second Bragg cell members are responsive to a radio frequency
signal that changes the index of refraction and provides
steering.
In a method aspect of the present invention, optical intersatellite
and satellite-to-ground communication in free space is facilitated.
The method comprises the step of frequency division multiplexing a
plurality of communication signals into a single, broad band
frequency division multiplexed signal. The method further comprises
the step of generating with a laser an optical carrier signal and
the step of phase modulating the optical carrier signal with the
broad band frequency division multiplexed signal by mixing the
multiplexed signal with the optical carrier signal in an
electro-optic modulator to produce a phase modulated optical
communication signal.
After phase modulation, the method further comprises the step of
transmitting the phase modulated optical communications signal to a
receiver where it is demodulated back into the broad band frequency
division multiplexed signal and then demultiplexed into the
plurality of communication signals.
With the present invention, the method can also include the step of
generating a plurality of digital communication signals and
modulating those signals into analog communication signals, also
known as digital waveform, before frequency division multiplexing.
These modulated digital communication signals can be up converted
and then combined with other up converted analog communication
signals into a single, broad band frequency division multiplexed
signal.
In accordance with another aspect of the present invention, a beam
steering device allows free space optical communications. The beam
steering device includes a Bragg cell having first and second Bragg
cell elements for providing two-dimensional indexing of the phase
modulated optical communication system. The first and second Bragg
cell elements are responsive to the input of a radio frequency
control signal. A liquid crystal display also receives the phase
modulated optical communication signal to provide fine
steering.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present invention
will become apparent from the detailed description of the invention
which follows, when considered in light of the accompanying
drawings in which:
FIG. 1A is a schematic block diagram of the first half of the
system of the present invention and showing band limited signal
sources that are later combined.
FIG. 1B is a schematic block diagram of the system of the present
invention and showing the frequency division multiplexer and laser
that generates an optical carrier signal and the beam steering
devices.
FIG. 2 is a schematic block diagram of a non-mechanical steering
device of the present invention having a liquid crystal display and
first and second Bragg cell elements.
FIG. 3 is an overall schematic block diagram of functional elements
used in the system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention may,
however, be embodied in many different forms and should not be
construed as limited to the embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be
thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like
elements throughout.
The present invention is advantageous because it now provides a
method and system of communicating in free space with an optical
carrier signal that is a phase modulated carrier and is
advantageous over intensity modulation. With the constant envelope
type of phase modulation, no auxiliary or pilot signal generator or
detector is required. Additionally, the distances that often change
between ground stations and satellites and/or two satellites in
space would not affect the modulated communication signal as
compared to an intensity modulated communication signal where the
distance changes could create inconsistencies in data transmittal
because of attenuation. Although some prior art systems disclose
frequency modulated optical communication systems, most of these
prior art systems are directed to the use of optical carrier
signals transmitted through fiber. The present invention also
allows a non-mechanical steering device that is advantageous
because fewer moving parts are required, which allows faster
reaction rates.
Referring now to FIGS. 1A and 1B, there is illustrated at 10 the
system of the present invention, which processes both digital and
analog communication signals within a frequency division
multiplexer and modulation unit indicated by the dotted line
configuration at 10a. Thus, the present invention is advantageous
because it allows both analog and digital communication signals to
be transmitted along the same optical carrier through free space,
such as in intersatellite communications.
For purposes of illustration, various examples of band limited
signal sources are illustrated. Four digital data sources, an
analog signal source, an RF receiver that receives analog signals
and a remote amplifier and filter that receives other analog
signals.
The sources of digital communication data 12a-d form an overall
source or means for generating a plurality of digital communication
signals. As illustrated, two of the data sources 12a and 12b are
low speed data sources and communicate to a time division
multiplexer 20 that receives the plurality of digital communication
signals and multiplexes the digital communication signals into a
plurality of time division multiplexed data streams. In the
illustrated example, the two sources 12a and 12b can include many
other sources (not illustrated) that are time division multiplexed.
The digital data signals that are multiplexed are illustrated as
low speed digital data channels (LSDC) and combined into moderate
data rate time division multiplexed data streams.
For purposes of illustration, the process of frequency division
multiplexing will be described relative to the low speed data
sources 12a and 12b. The time division multiplexed data streams are
then independently encoded using forward error correction (FEC) 22
and then pass through respective digital modulators 24 to produce a
wave form of an analog signal as known to those skilled in the art.
The modulated signals then enter the frequency division multiplexer
(FIG. 1B), which includes respective mixers 28 that up converts
each analog communication signal into a respective frequency slot
or channel (shown as channels 1, 2 . . . N) at a typically higher
frequency. This is accomplished through respective coding input to
the mixers as illustrated by coding input lines indicated at
F.sub.1 through F.sub.m. Those up converted frequencies then pass
through a bank of band pass filters 30, and then into an N-way
combiner 32 where the signals are combined into the frequency
division multiplexed signal.
Depending on one skilled in the art, modulation choices can be
selected depending on the design preferences. It is possible to
have different codings, including concatenated codes and
interweaving. A convolution code and a standard Reed-Solomon code
could also be used adequately in the present invention for FEC. Any
higher speed base band digital data channels (HSDC), such as from
sources 12c and 12d, do not pass into the time division multiplexer
because of impacts on spacecraft prime power and hardware overhead.
Thus, these signals pass through the forward error correction 22
and are digitally modulated by modulators 24.
Analog signal source 12c passes through modulator 24, but not
forward error correction. Other analog wave forms, such as received
on an uplink, but not demodulated on-board, or other frequencies
are multiplexed without passing through any forward error
correction and modulators. These signals can be received in an
antenna 33 of an RF receiver 33a, or pass into an antenna 33b and
pass as a remote RF signal to an amplifier and filter 33c. All
communication signals are subject to multiplexing as described
before to produce a broad band frequency division multiplexed
signal. The steps of multiplexing includes up converting each
signal to a desired, unique frequency, then band pass filtering
those up converted frequencies and combining the total up converted
frequencies into a broad band frequency division multiplexed
signal. The frequency division multiplexer can be made flexible by
introducing dynamic channel and bandwidth allocations so that a
laser producing the optical carrier signal will have a bandwidth
that can be allocated "on-demand" by programming local oscillators
and filters (not shown).
As shown in FIG. 1B, a transmit laser 34 generates an optical
carrier signal. An electro-optic modulator 36 receives the broad
band frequency division multiplexed signal and the laser generated
optical carrier signal and phase modulates the optical carrier
signal with the multiplexed signal to produce a phase modulated
optical communication signal. As known to those skilled in the art
in phase modulation, the instantaneous phase of the optical carrier
signal is shifted in accordance with the modulating signal. In
phase modulation, the extent of the phase shift is directly
proportional to the amplitude of the modulating signal. The
rapidity of the phase shift is directly proportional to both the
amplitude and the frequency of the modulating signal, thus
distinguishing phase modulation from frequency modulation where the
result is a difference in the frequency-response
characteristics.
The electro-optic modulator 36 of the present invention can
comprise a Mach-Zender electro-optic modulator. This final
up-conversion to "light band" as described is performed using the
external analog modulation and power amplification techniques
similarly used with continuous wave outputs of microwave
transmissions. External modulation of the laser allows much wider
modulation bandwidths than with direct laser diode modulation.
Further, external modulation allows the laser diode to be selected
without regard to the required modulation bandwidth using a
Mach-Zender modulator and allow a very wide bandwidth (20 GHz). The
laser requires low power and the Mach-Zender modulator can also
implement frequency modulation besides the desired phase
modulation.
After phase modulation, the phase modulated optical communication
signal can pass through a sideband filter and then into
erbium-doped fiber amplifier 40. The desired optical carrier signal
produced by the laser 34 is about 1,550 nm to allow amplification
of the phase modulated optical communication signal with
erbium-doped fiber amplifiers.
The amplified signal then passes into beam processing optics 50 and
through a beam steering device 52 of the present invention. As
illustrated in FIG. 2, the beam steering device 52 includes a Bragg
cell 54 formed from first and second Bragg cell elements 54a, 54b
that provide for two-dimensional "coarse" steering or indexing
followed by a liquid crystal display 56 that allows for fine
indexing or steering of the phase modulated optical communication
signal. The first and second Bragg cell elements 54b are connected
to respective radio frequency sources that are highly stable,
spectrally pure, local oscillators that are used for steering the
optical communication signal from a ground station or via
satellite.
As illustrated, a receiver 70 can be positioned in a satellite to
receive the phase modulated optical communication signal. The
receiver 70 includes a beam steering device 72 having a structure
similar as in beam steering device 52. Beam steering device 72
receives the signal and then passes the signal to beam processing
optics 74 and optical amplifier 76. The signal passes to an
optical-to-electrical PM or FM demodulator 78 (i.e., optical phase
locked loop demodulator) that receives VCO from receiver laser
source 79. The signal passes to N-way splitter 80. The individual
channels from the N-way splitter that correspond to each signal
pass through band pass filters 82 and respective mixers 84 that are
subject to coded sequences F.sub.1 through F.sub.M. The individual
signals after demixing pass through low pass filter 86 and form the
signal sinks 1-N 88 as illustrated. Because phase locked loop
detection can be used, the Doppler shift induced by the relative
motion of two communication platforms, such as in intersatellite
communication systems, can be compensated.
After phase locked loop demodulation, as described above, the broad
band frequency division multiplexed signal passes through the N-way
splitter 80 where the various channels are separated in the basic
reverse sequences of steps as described in the modulating and
frequency multiplexing steps. The individual receiver would also
maintain the bit and frame synchronization for proper decoding of
the data streams. Naturally, appropriate circuitry can act as a
digital demodulator for producing original digital data
streams.
FIG. 3 illustrates a high level block diagram of a laser
communications terminal indicated at 100, which can be used such as
in a satellite. As noted, the optical carriers are generated by the
laser for transmit and receive laser beams in the 1,550 nm band and
offset by several nm to ease optical filtering. Point and tracking
(PAT) beacons 102 for the laser 34 can use wavelengths less than
one micrometer. All optical signals are envisioned to share the
same telescope optics. The interfaces to the optical modulator and
detector portions of the overall terminal are designed to mimic a
typical microwave transmitter. Therefore, microwave links can be
upgraded transparently. The laser beam 34 is typically a
semiconductor laser diode and chosen for wavelength considerations.
The beam processing optics is a collection of filters,
splitter/combiners, lenses and collimators that are used to deliver
coherent beams to proper destinations as known to those skilled in
the art.
The point and tracking sensor and signal processing circuit 104 are
used for the initial acquisition and, if necessary, reacquisition
in tracking of various transmit and receive laser beams for
communication with another satellite or spacecraft. Special
telescope pointing tracking beacon circuits 106 are used for
optically establishing and maintaining optical alignment between
two satellites or platforms. The non-mechanical steering device 52
of the present invention is used to mimic the usual mechanical
devices found in some steering assemblies. The Bragg cell can also
provide for beam spoiling where the beam divergence is
intentionally increased to search the entire field of uncertainty
(FOU) for a receiver located on another satellite. The usual
approach is to perform a spiral scan of the field of uncertainty,
which is time consuming and necessitates spacecraft attitude
compensation.
The optical method of the present invention requires no
compensation and the optical approach using the liquid crystal
display and Bragg cell decreases the payload weight, requires less
fuel, and dramatically decreases acquisition times. It also
provides higher bandwidth controls to maintain pointing, which
reduces burst errors. Other circuits that are known to those
skilled in the art are the optical detector circuit 106, the
frequency division multiplexer transmitter 10a, the receiver 70
having the frequency division demultiplexer, the telemetry I/F and
power conditioning circuit 110, and the system controllers and
processing circuits 112. An appropriate circuit bus 114
interconnects the various circuits.
The point-ahead angle for the laser beam can be calculated using a
fast read-out focal plane CCD array and a two spot system. The
point-ahead angle can be derived by computing the Euclidean
distance between the centroids of a receive beacon and transmit
signal. The data can be processed using a specialized read-out
algorithm and dedicated digital signal processing hardware.
It is evident that the present invention is advantageous because it
now allows both data and analog communication signals to be
transmitted on an optical carrier signal through phase modulation.
The drawbacks of intensity modulation are also overcome by the
present invention. The non-mechanical steering device of the
present invention also is advantageous to allow greater bandwidths
and faster tracking times.
Many modifications and other embodiments of the invention will come
to the mind of one skilled in the art having the benefit of the
teachings presented in the foregoing descriptions and the
associated drawings. Therefore, it is to be understood that the
invention is not to be limited to the specific embodiments
disclosed, and that the modifications and embodiments are intended
to be included within the scope of the dependent claims.
* * * * *